Allow overriding the fast paths
Allow overriding the fast paths

For benchmarking purposes we should allow overriding the fast path
selection through the definition of two symbols. First, we need a
`GRAPHENE_FORCE_USE` symbol to disable the selection done when
generating the graphene-config.h header at configure time; this
allows the user to define the appropriate `GRAPHENE_USE_*` symbol
prior to including the main graphene.h header.

ebassi/graphene,criptych/graphene,criptych/graphene,ebassi/graphene,criptych/graphene,criptych/graphene